ABSTRACT The goal of this F31 resubmission is to investigate the mechanisms of T cell activation and the consequences in the deadly syndrome of heart failure (HF), currently the leading cause of mortality and hospitalizations in the USA. Using the well- established mouse model of HF induced by transverse aortic constriction (TAC), we and others have demonstrated a central pathogenic role for T cells in cardiac remodeling and function. However, the specific mechanisms of T cell activation in HF remain unknown. Classic T cell activation occurs upon antigen presentation by antigen presenting cells, including dendritic cells (DCs), B cells and macrophages, through the T cell receptor (TCR). Emerging evidence suggests that T cells can also be activated by alarmins, soluble inflammatory mediators, through TCR independent pathways in a ?non classic manner. My preliminary data using the TAC model in wild type and ovalbumin (OVA) immunized OT-II transgenic mice, in which classic T cell responses are limited to OVA, indicate that both classic and non-classic T cell activation occur and can separately lead to cardiac fibrosis (CF), but are not sufficient to induce cardiac dysfunction. The triggers (specific cardiac antigens or alarmins), the timing and the location of these T cell activation pathways and the APC involved during the progression of HF remain unknown. Based on these data, I will test the central hypothesis that both classic and non classic T cell activation mechanisms cooperate to initiate and sustain pathological cardiac remodeling and cardiac dysfunction during the progression of TAC induced HF. I will test this hypothesis in two specific aims (SA). SA1 will determine the specific location and timing of classic T cell TCR activation and the APCs triggering this response during TAC using Nur77GFP mice, which transiently express GFP in response to TCR stimulation (SA1a). I will also identify the TCR clonotypes that emerge during the progression of TAC induced HF by TCR sequencing of single GFP+CD4+ T cells sorted from the heart of TAC mice (SA1b). In SA2 I will perform adoptive transfer experiments of WT and alarmin sensing-impaired effector CD4+ T cells into mice normally protected from HF and evaluate cardiac remodeling and function. I will further investigate the alarmin induced mechanisms of T cell dependent CF in vitro (SA2b). The therapeutic potential of depleting CD4+ T cells after TAC-induced cardiac remodeling has occurred will also be evaluated (SA2c). Completion of these aims with the support of my mentor and collaborators will result in a deeper understanding of novel mechanisms that control the timing, and progression of the T cell immune response in pathological remodeling of the heart, while supporting my training potential and PhD candidacy.